In the production of cellulose pulp from comminuted fibrous material, preferably wood chips, by chemical methods, for example kraft cooking, there is a need to pretreat the material before the cooking in order to drive out gas and vapour from the cavities inside the wood chips and replace it with a liquid at the same time as the chips are heated.
Conventional pretreatment processes normally include a chip bin from which the chips are fed via a low pressure feeder to low pressure system which comprises a steaming vessel followed by a chip chute. At the lower end of the chip chute there is optionally arranged a pump and thereafter a pressure sluice, a so called high pressure feeder, by aid of which the chips are introduced into a high pressure system. From the high pressure feeder there is also an outgoing line that establishes a liquid phase low pressure circulation together with a pump, a sand trap, a tubular strainer and a level tank. By this low pressure circulation, circulating liquid is transferred back to the fibrous material in an upstream position. The high pressure system includes a transfer circulation that transfers the chips, together with liquor, to the top of a digester or an impregnation vessel. A part of the liquor is separated from the chips at the top of the digester and transferred back to the high pressure feeder. The chips are cooked in the digester in an upper concurrent zone followed by a lower countercurrent zone. An extraction screen section is arranged between these zones. Hot black liquor is extracted from this screen section and led to a flash tank, which is usually coupled in series with a second flash tank. The steam phase from the flash tanks is usually used as a direct heat source in said chip bin and/or steaming vessel. There is a need to cool (by means of cold white liquor or separate cooling with cold water) the liquor in the return line of the transfer circulation and/or the circulating liquid in the low pressure circulation in order to avoid flashing and resulting bangs in the high pressure feeder.
From U.S. Pat. No. 5,053,108 there is known a process where black liquor is led to the chip chute. This black liquor originates from the flash tanks and has hence been cooled through the pressure release in the flash tanks.
In a conventional system there is a potential to enhance the heat economy to an extent which has not been fully done in the past.
The object of the present invention is to provide an improved method in connection with the pretreatment of comminuted fibrous material, which method provides a way to avoid bangs in the high pressure feeder and also gives better heat economy and less equipment than conventional methods.
This is achieved by the method according to patent claim 1, which means that hot process liquid and/or circulating liquid in the low pressure circulation is evaporatively cooled by being allowed to flash within said low pressure system. Said hot process liquid is preferably uncooled before its entry into the low pressure system. The pressure of said hot process liquid is preferably essentially the same as in the high pressure system.
Normally, said low pressure system includes a low pressure circulation where liquid is withdrawn from said fibrous material and is recirculated back to said fibrous material in an upstream position. The conventional low pressure circulation comprises a level tank for pressure balancing against a gas phase part of the low pressure system, so that a liquor level (normally in the chip chute) between said gas phase part and said liquid phase part can be controlled and a liquid flow from said low pressure circulation can be stabilised. Said level tank is in the following included in the low pressure system and in the low pressure circulation, although the liquid which is formed in the bottom of the tank is normally withdrawn from the circulation and also although the saturation pressure at the temperature in the level tank may be lower than in the rest of the low pressure system. According to a further aspect of the invention said hot process liquid and/or said circulating liquid is allowed to flash into said level tank.
A further aspect of the invention is that steam is produced by said flashing and that this steam is used for direct heating of said fibrous material.
A further aspect of the invention is that said hot process liquid comprises black liquor.
Yet a further aspect of the invention is that said hot process liquid has a temperature of 100-180.degree. C., preferably 110-170.degree. C. and more preferably 120-160.degree. C. and a pressure of 3-25 bar, preferably 4-20 bar and more preferably 5-15 bar.
According to a further aspect of the invention, the pressure in said low pressure system is 0.5-5 bar (abs), preferably 0.7-4 bar and more preferably 1-3 bar and the temperature is 60-145.degree. C., preferably 80-130.degree. C. and more preferably 90-120.degree. C.
Yet a further aspect of the invention is that a liquid phase is formed in said level tank, which liquid is conveyed to said transfer circulation of the high pressure system in order to be further conveyed to the digester.
Another advantage of the method is that flashing with resulting bangs in the high pressure feeder can be avoided without the need of energy degrading cooling by cold water.
An advantage of the method according to the invention is that the live steam consumption can be reduced which results in an improved heat economy.
Another advantage is increased dryness of spent black liquor.
Another advantage is that less equipment is needed when performing the same process as in conventional systems.